It is now well-known to utilize a wide-baseline, or wide-aperture, array of sensors, such as in a passive sonar system, to measure the range from the sensor array to an emitting source. While range can be determined by echo timing in the active case, in the passive case no echo exists and so range must be calculated from the curvature of the arriving sound energy wavefront. Differences in some characteristic of this arriving sonic wavefront must be measured with a high degree of accuracy; this is equivalent to finding the "best focus" with range as a variable parameter. Normally, only three sensors are required for estimating this curvature and thus finding the range to the source, assuming that the sensors lie at known distances along a curve with a precisely known set of characteristics; most typically, the sensors lie at equal distances along a straight line, for simplicity of calculation. However, it is well-known that minor deviations from a straight line can be expected due to various stresses and strains placed upon the structure supporting the three sensors, especially when that structure is in a dynamic condition, e.g. the hull of a vessel moving through the water. Thus, where a trio of sonar sensors are placed upon a ship hull, and used to passively receive sonar information for estimating the range of the sonar source, relatively small flexure of the hull can result in relatively large range estimation errors. It is therefore highly desirable to provide a method of, and apparatus for, estimating flexure of a substantially-rigid body holding a trio of sensors, and for using the flexure information thus obtained to reduce potential error in the resulting measurements.